1. Field of the Invention
An image forming apparatus, such as a copying machine, a printer, a facsimile apparatus, etc., forms images on both sides of a recording medium using a one-path method, and a control method of the image forming apparatus.
2. Discussion of the Background
As methods of forming images on both sides of a recording medium, such as a transfer sheet, a switchback method and a one-path method are known. Japanese Patent No. 3484996 describes an example of the one-path method. In the switchback method, an image is formed on one side of a recording medium by causing the recording medium to pass a transfer device and then the recording medium passes a fixing device so that the image is fixed to the recording medium. Thereafter, the recording medium is returned to the transfer device and the fixing device so that another image is formed on the other side of the recording medium.
In the one-path method, visible images are transferred onto both sides of a recording medium by a both-sides transfer device, and thereafter the recording medium passes a fixing device, so that the images on both sides of the recording medium are fixed to the recording medium at the same time. The one-path method is superior in cost to the switchback method because a relatively complicated switchback mechanism is avoided.
Further, the time for forming the images on both sides of the recording medium is shorter in the one-path method because the recording medium is not returned to the transfer device and the fixing device after forming the image on one side thereof. Furthermore, in the switchback method, when the recording medium is curled by the heat of the fixing device and the curled recording medium is returned to the transfer device and the fixing device, the recording medium tends to jam. Such jamming of the recording medium is avoided in the one-path method.
In image forming apparatuses forming images on both sides of a recording medium using the one-path method, a known method uses a common latent image bearing member for forming the image on one side of the recording medium and for forming another image on the other side of the recording medium. Another known method uses separate latent image bearing members for forming the image on one side of the recording medium and for forming the other image on the other side of the recording medium.
In the method that uses the common latent image bearing member, a first latent image is formed on the latent image bearing member by optically scanning the latent image bearing member, and a first visible image is obtained by developing the first latent image. Then, the first visible image is transferred onto an intermediary transfer member, such as an intermediary transfer belt, etc. Subsequently, a second latent image is formed on the latent image bearing member, and a second visible image is obtained by developing the second latent image. Then, while the second visible image is transferred onto the backside of the recording medium, the first visible image transferred onto the intermediary transfer member is transferred onto the front side of the recording medium.
In the method that uses separate latent image bearing members, while a first visible image formed on a first latent image bearing member is transferred onto the front side of the recording medium, a second visible image formed on a second latent image bearing member is transferred onto the backside of the recording medium. In this method, because visible images for the front side and the backside of the recording medium are formed substantially simultaneously, the image formation speed is faster as compared to the method that uses the common latent image bearing member.
In the above-described method that uses separate latent image bearing members, however, there is the possibility that a deviation from a predetermined position is caused to the images on both sides of the recording medium in the conveyance direction of the recording medium. Generally, in an image forming apparatus, a main controller including a CPU, a RAM, a ROM, etc. controls a driving of a latent image bearing member, a development device, a transfer device, a sheet conveying device, etc., and a latent image writing device writing a latent image on the latent image bearing member is controlled by a separate writing controller because of the necessity of processing a large volume of information.
The main controller determines a time criterion in a series of image forming operations upon receiving an image formation instruction inputted by an operator, and based on the time criterion, drives the development device, the transfer device, etc. at appropriate timings. Further, the main controller transmits a writing instruction signal to the writing controller to determine the timing of starting a writing process. At this time, the main controller not only performs a calculation process for counting the timing for transmitting the signal of the writing instruction but also performs calculation processes for driving the development device, the transfer device, etc. at the same time.
In this case, if the calculation process for determining the timing for starting the writing process is interrupted by the calculation processes for driving the development device, etc., the timing for transmitting the signal of the writing instruction to the writing controller may be deviated from a regular timing. When (i) the timing for transmitting the writing instruction signal to the writing controller controlling writing of the latent image for the front side of the recording medium and (ii) the timing for transmitting another writing instruction signal to the writing controller controlling writing of the other latent image for the back side of the recording medium are irregularly deviated from regular timings, respectively, a deviation in the positions of resulting images is caused on both sides of the recording medium in the conveyance direction of the recording medium.
For example, the first character line of a first page of a document formed on the front side of the recording medium and the first character line of the second page of the document formed on the backside of the recording medium are not aligned with each other.
The above-described JP No. 3484996 describes a technology for suppressing the deviation in the positions of the images on both sides of the recording medium in the widthwise direction (the direction perpendicular to the conveyance direction) of the recording medium. However, JP No. 3484996 fails to disclose a technology or method for suppressing the deviation in the positions of images on both sides of the recording medium in the conveyance direction of the recording medium.